1. Field of the Invention:
The invention relates in general to a fuel pellet for a fuel rod of a fuel assembly of a nuclear reactor core and in particular to a fuel pellet having fissile, fertile, and burnable poison material disposed at predetermined radial locations within the fuel pellet.
2. Description of the Prior Art:
To generate a predetermined amount of energy, a fuel assembly is loaded with fuel rods which include pellets enriched in fissile material (for instance, UO.sub.2 pellets enriched in U.sub.235). The nuclear reactivity (known in the art as K which is defined as the number of fissions in one generation divided by the number of fissions in the preceeding generation) of such an assembly is highest at the beginning of life and lowest at the end. To produce a certain fission rate at the end of life, excess enriched material is provided at the beginning of life. The reactivity of the assembly must be kept within tolerable limits at all times. The most limiting condition is early in the life of the core, since the reactor control system may not have sufficient nuclear worth to maintain the core subcritical by a certain margin in the cold condition due to the high reactivity of the fuel. The reactivity of the fuel assembly can be controlled by fuel rods having fuel pellets containing a burnable poison dispersed therein. The burnable poison is added in sufficient quantity to suppress the reactivity of the fuel to a level consistent with the capabilities of the reactor control system so that the reactor safety design criteria can be met. The burnable poison fuel rods are located in the interior of the fuel assembly so that the burnable poison does not become prematurely depleted, and so that the nuclear interaction between the control rods in the reactor and the burnable poison fuel rods is minimized. The fuel pellets containing the burnable poison are a homogeneous mixture of fissile and fertile nuclear materials and burnable poison materials. The powders containing these materials are blended and mixed in a manner to promote maximum dispersal of the materials into a homogeneous mixture. The burnable poison material tempers the excess reactivity of the enriched materials by absorbing neutrons throughout the lifetime of the fuel assembly. However, it is only necessary to restrain the excess reactivity of the fuel assembly near the beginning of the fuel cycle. Thereafter the excess burnable poison materials that are necessary to temper the reactivity of the fuel assembly near the beginning of the cycle continue to decrease the reactivity and the power output of the fuel assembly throughout the lifetime of the fuel assembly long after the beneficial control at the beginning of the cycle is necessary. The neutron absorption strength of the burnable poison is proportional to the concentration of the burnable absorber and the surface area of the absorbing material. Under the teachings of current state of the art, the concentration of the burnable neutron absorber at the beginning of life of the fuel assembly is selected so that enough burnable poison atoms are available in the proximity of the absorbing surface to maintain the reactivity of the fuel assembly below a predetermined value at some point in time in the life of the fuel assembly. For example, for boiling water reactors, the most limiting condition from a nuclear safety design point of view occurs after a few months of operation in the first cycle, somewhere towards the middle of the cycle. The rate of burnable poison depletion is controlled by increasing or decreasing the number of fuel rods which include the poison atoms (i.e. increasing or decreasing the absorbing surface area). Accurate and optimized control of reactivity is difficult to achieve. As the number of burnable poison fuel rods is increased while decreasing the absorber concentration per rod (a situtation which favors rapid depletion of the poison and reduced end of cycle reactivity penalties) the power distribution within the assembly early in life becomes more distorted as the power generated in the peak rod increases relative to the average power in the assembly, trending towards unacceptable regions based on thermal hydraulics and nuclear design and safety considerations. Alternatively, as the number of burnable poison fuel rods is decreased (i.e. the burnable poison atoms are concentrated at few locations) the depletion rate of the poison is reduced and the end of cycle residual poison reactivity penalties are increased. These penalties are compensated for by increasing the enrichment of fissile material within the fuel assembly which, accordingly, corresponds to higher costs for the fuel assembly. Therefore it would be desirable to have a means for minimizing the amount of burnable poison material that is necessary to be disposed within the fuel assembly and for controlling the reactivity of the fuel assembly without increasing the number of burnable poison fuel rods. It would further be desirable to have a means to control the reactivity of the nuclear fuel assembly without the use of burnable poison material.